Experiment 8: Semiconductor Devices

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1 Name/NetID: Experiment 8: Semiconductor Devices Laboratory Outline In today s experiment you will be learning to use the basic building blocks that drove the ability to miniaturize circuits to the point that now you have a computer inside a small mobile device that has more computing power than the flight computers inside the early space-flight missions the diode and transistor. The miniaturization was achieved through the use of semiconducting material whose conductivity could be varied easily at a very small scale. Because the behavior of both are both notoriously nonlinear, we will first focus on characterizing these new devices and next week we ll apply them as a key feature in motor-speed control. Throughout this experiment, we ll employ simple models of both the transistor and the diode. It is important to realize that in circuit analysis we employ idealized models of every device and the choice of each model represents a tradeoff between utility and accuracy. Learning Objectives Measure, analyze, and model the IV characteristics of the diode. Characterize the motor using a BJT-based motor-drive circuit. Recognize the efficiencies of using this design over that of voltage-divider-based motor control. Teammate/NetID: Section AB/BB: A B C D E F (circle one) Modelling Diodes: The diode can be thought of a one-way valve that only allows current to flow in one direction. With this capability we can build circuits that regulate voltage or protect devices from being powered incorrectly. In this lab we ll examine what this behavior means in terms of the IV characteristics of the device. In addition, we ll develop a linearized model of the diode in the same way we did for the motor last week. Building Circuit with Transistors: The transistor is a three terminal device that can be thought of as an electrically-controlled switch. In today s experiment, you ll learn to construct simple circuits that employ the transistor as a switch for controlling the current flow through a branch of a circuit.

2 Diodes Like many devices, the diode is best characterized by the voltage current relationship of the device. In previous labs, we examined the IV curves of resistors and motors. Today, we ll examine the IV curve of a diode and use that to develop some intuition on how diodes behave. Before we characterize the device, let s take a moment to look at the symbol, device packaging and some basic terminology so that we minimize confusion. Think of the symbol as an arrow that indicates which direction it will allow current to flow (current flows from anode to cathode). Basic diodes have two states in which they operate. The diode is referred to as being ON when current is flowing through the device. The diode is referred to as being OFF when current is not allowed to flow. Each diode has a turn-on voltage which indicates the minimum voltage necessary to turn the diode ON. Figure 1: Anode and cathode of a diode Now, let s analyze the behavior of a diode within a very simple circuit. Construct the circuit shown in the 2 using the DC power supply as the voltage source. Setup both DMMs to measure the voltage across the diode and current through the diode simultaneously.

3 Figure 2: Simple circuit for collecting IV characteristics of a diode. R is a current-limiting resistor, typically around 330 Ω. The voltage source is a DC power supply. Sweep the voltage of the power supply from 0V to 7V and collect the voltage across the diode and current through the diode. Idea: Instead of using an ammeter, you can record the voltage across the resistor as well as the diode voltage and compute current using Ohm s Law. V S V D I Notes Question 1: Table 1: IV data for a diode, forward biased.

4 Now take a single measurement at V = 3V. (You can do this by reversing the polarity of the power supply.) Make sure to label your voltage as negative in the table! Voltage Current Notes Question 2: Table 2: IV data for a diode, reverse biased with V = 3 V. Question 3: Plot your diode data, I vs. V D, in MATLAB as a single IV curve. Question 4: Perform a linear curve fit on the region in which the diode is on (don t assume the OIM model, that is, don t fit a perfectly-vertical line to it!). Estimate the voltage at which the diode turns ON. Question 5: Using the linear curve-fit, determine the Thevenin equivalent circuit of a diode when it is on. Transistors The advantage of using diodes in a circuit is that it allows us to control the direction of current flow through the circuit. Transistors give us even greater control over current flow through the circuit. As with the diode, the pins on the transistor serve specific functions and correspond to specific nodes on the schematic symbol. A quick reference is shown below to help with wiring up your transistor circuit.

5 Figure 4: Pinout of the transistor used in ECE110 lab. In general, the transistor symbol is drawn without the letters B, C and E explicitly written on each of the nodes. Instead, an arrow is drawn coming out of the emitter (as shown) leaving the Collector and Base easily distinguishable. It should also be noted that for the purposes of this course, the arrow on the transistor indicates the direction in which current flows through the device. Modelling the Transistor as a Switch The transistor can be modeled as a simple voltage controlled switch where the Base of the transistor is the input and the Collector to Emitter connection is either an open circuit or a closed circuit. Reference Figure 5 below. When the input, V S, is 0 V the switch is OFF and current is not allowed to flow through the device. The collector seems to be open-circuited from the rest of the transistor. When the input is, for example, 5 V, the switch is ON and current is allowed to flow into the collector. The switch has changed such that the collector seems to be connected. With that in mind, let s construct the following circuit where a transistor is used to control the current flowing from a battery through a 100 Ω resistor.

6 While this notional description of the transistor is helpful for gaining some intuition about the circuit, the transistor itself is commonly considered be controlled by current flow. This is why we have a 330Ω resistor from the power supply to the base to limit the current flow into the input. Figure 5: Simple circuit for collecting IV characteristics of a transistor. Question 6: Sweep the voltage of the power supply from 0V to 5V and record several data points of the voltage across the 100 Ω resistor and the corresponding voltage across the collect/emitter. Make a table below and place these values in it.

7 Question 7: Plot the voltage across the 100 Ω resistor as a function of the input voltage, V S. Indicate the region of input voltages over which the transistor switches at the collector from an open circuit to a closed circuit. HINT: pay attention to the lower values of V S! The real advantage of using the transistor in this manner is that we can now use circuitry powered by one voltage source to control circuitry powered by a secondary voltage source. Traditionally, this type of circuit is called an amplifier circuit because we can use a small power input to control a much larger power source. As you may have seen from your own experimenting, the output pins on the Arduino have a hard time powering our DC motors. By replacing the 100 Ω resistor (in the above circuit) with a motor, we can control the motor while allowing it to draw current directly from the battery. Driving a Motor The circuit shown below is basic motor drive circuit that we will use for the remainder of this semester. As you can see, this circuit makes use of both a transistor and diode to drive the motor. As hinted in the previous section, the transistor is used to switch the motor on and off. In addition, we ve added a diode in parallel with the motor to protect our circuitry from any spurious voltage spikes generated by the motor. Figure 6: Circuit schematic of the motor drive circuit. Construct the motor drive circuit detailed in Figure 6 using the DC power supply as the input voltage and the NiMH battery as the 7.2V source. Add an ammeter to measure I C.

8 Question 8: and I C. Sweep the DC power supply V in from 0 V to 5 V and measure both the voltage across the motor V m Comments: Table 3: Motor-drive circuit Characteristics

9 Question 9: For a data point near V m 4 V, use the value of I C to model the motor as a resistor. For this data point, what resistance, R m, is a reasonable model for the motor? Show your work. Record this in your Lab 9 Prelab! R m = Question 10: Make a note of the turn-on voltage V in and corresponding collector current I C and record them here. Question 11: At what input voltage V in does the motor hit its maximum speed? i.e. When does the motor speed stop increasing? Does this match the point at which I C no longer increases? NOTE: This happens due to an effect of the transistor circuit called saturation. Question 12: Starting from 5V, slowly turn down the input voltage V in until the motor stalls. Record the voltage. Question 13: Set the DC power supply to 5V and measure the total current sourced by the power supply as well as the voltage drop across the motor. Assuming the power delivered by the DC power supply is negligible compared to that of the battery, compute the efficiency of the circuit. That is, compute the ratio of the power absorbed by the motor divided by the total power delivered by the battery. In next week s experiment, we ll explore a method of controlling motor speed that requires this circuit. You may wish to leave this circuit assembled on your breadboard so you won t need to rebuild it next week.

10 Conclusion Question 14: Briefly discuss the process of modelling the diode and the advantages or disadvantages of the model you developed. Question 15: Explain how the transistor aids in driving a motor. In your explanation, describe what challenges would occur if the transistor was not involved. While the models we used for diodes and transistors are highly simplified, they still have a great deal of utility. In particular, these devices give us control over the current flow through the branches of a circuit. This is particularly useful when we want to control the current through a motor using a controller that can t handle a great deal of current itself. Next week, we ll gather some of the final pieces of the puzzle necessary for building a power-efficient autonomous car. What You Learned After today s lab, you are able to build circuits involving simple semiconductor devices. You understand the tradeoffs made when modelling semiconductor devices and how those models can be employed in circuit design. Lastly, you can build a highpower circuit necessary for driving a small DC motor via a low-power control signal. Explore More! As time permits, please continue your experimentation using the Modules recommended for this week or other modules from previous weeks. Recommended are Explore More! Clipping Circuits, Explore More! Arduino-based Wall-Follower, and Explore More! Characterizing Optical Sensors.

11 Lab Report Rubric The following rubric will be provided at the end of each lab procedure. As a final step in preparing your lab report, you will use this rubric to analyze your own performance. Section Criterion Comments: Experimental Setup and/or Design Description Measurements Computations Circuit Schematics are drawn neatly, accurately, and properly labeled. Decisions regarding experimental setup and design are clearly explained. Tables include units and proper precision. Any new device introduced should be characterized using measurements! Computations performed on raw data are explicitly described and follow rules for significant figures. Analysis Modeling Conclusion General Formatting Self-assessment Graphs have title, labels, units, scale, legend; Lines for curve-fitting appear in the graph when needed and parameters like the intercepts and the slope are labeled. A mathematical model for the curve-fit graph allows for more abstract references to the device s behavior. The expected behavior is explained in the context of the graph. Conclusions are drawn from your experimental results to support the reason(s) for completing the experiment. Closes the loop on the Introduction. Answers to questions clearly labeled. The overall appearance of the report is professional. This table has been thoughtfully completed.